Method to form hot cathodes



United States Patent 0 U.S. 'CL 117-223 2 Claims ABSTRACT OF THE DISCLOSURE A cathode sleeve and emissive coating are heated rapidly to about 1,200 C. with the heating being repeatedly interrupted to maintain a maximum temperature difference between the sleeve and coating to minimize evaporation of the coating.

This invention relates to a method for forming hot cathodes of electron discharge devices having an emissive coating of alkaline-earth metal oxides.

In a known forming method, the cathodes are heated for approximately five minutes to a temperature of 1,000 1,200 0, either with or without drawing current and thereafter the temperature of the cathode is reduced in steps to the operating temperature within approximately 30 minutes. In this method a particularly large evaporation of barium occurs due to the high temperature and the length of time at which the temperature continues. The evaporation of the emissive coating causes faults in the insulation and a reduction of emission. When using a low starting temperature the barium evaporation is reduced. However, this requires excessive time, which is not economical for mass production.

In order to avoid these drawbacks, according to the invention, the cathode sleeve is heated in approximately 2-10 seconds, depending on the heat inertia caused by the material of the sleeve, to 1,200 C., in order to obtain a maximum difference between the temperature of the cathode sleeve and the temperature of the emissive coat. The heat is thereupon interrupted and switched on and off repeatedly up to a few hundred times.

Due to the sudden heating of the cathode, a high temperature gradient is obtained between the cathode sleeve and emissive coating as well as within the emissive coat from the end of the sleeve to the surface of the coat. The sleeve, having reducing means such as magnesium, carbon, or silicon for the alkaline-earth metal oxides, reaches a very high temperature so that these metals quickly diffuse into the coating, while the interior and surface of the emissive coating remain correspondingly colder,

Patented thereby limiting the evaporation of barium set free by the reducing means. Due to the saturation of available barium in the emissive coating, emission is obtained in a short time, which was not possible in the prior art. In some special cases, the sleeve should be cooled down to 300 C. before switching on the heat again.

Due to the rapid switching of the heat on and off, the cathode sleeve expands in all directions. This expansion causes the holes in the mica-discs in which the cathode sleeve is supported to be forced open. The cathode sleeve, adjacent the holes, deforms inwardly so that the crosssection of the sleeve exactly corresponds to the form of a hole in the mica-disc. At the high forming temperature the cathode sleeve applies soft pressure to seat in the holes of the mica disc, while during operating temperatures a natural fitting seat is obtained.

This avoids the variations, pressure and chattering that normally occur due to poor fittings about the cathodes during operation of devices, formed according to prior methods. No play occurs between the cathode sleeve and the hole in the mica disc. There is also no excess pressure since the heat emanated at the sleeve end is smaller and the temperature of the cathode is higher and more uniform over its entire length. In addition, the method provides an extremely long life expectancy for the cathodes. Also, filament failures, which would occur after a short time of operation, are now found and eliminated when forming the cathode, as a result of the rapid switching.

What is claimed is:

1. A method for forming hot cathodes of electric discharge devices having a cathode sleeve and an emissive coating of alkaline-earth metallic oxides, comprising rapidly heating the cathode to about 1,200 C. within 2 to 10 seconds, and repeatedly interrupting the heating for a sufficient time by switching the heat on and off from onehundred to several hundred times to provide a maximum temperature difference between the cathode sleeve and emissive coating to minimize evaporation of the emissive coating.

2. The method of claim 1 wherein the sleeve is cooled down to a predetermined lower temperature before the heat is switched on again to provide said temperature difference.

References Cited FOREIGN PATENTS 7/1926 Great Britain. 12/ 1927 Great Britain.

WILLIAM L. JARVIS, Primary Examiner. 

